Hollow beads are already known. Those made of organic material generally have the disadvantage of being destroyed at temperatures above 300.degree. C. Glass hollow beads generally melt at temperatures around 600.degree. C. and siliceous fly-ash hollow beads are thermally stable to above 1000.degree. C. However, glass and fly-ash hollow beads generally have a bulk density which is not easily controlled and is too high at over 100 gl and frequently have a chemical composition which is difficult to control.
Hollow beads have now been found which are characterized in that they contain aluminium phosphates or aluminium boron phosphates.
Hollow beads of the invention have, depending on any decomposition and/or oxidation processes of accompanying materials during production, a colour which varies between colourless, pale yellow via brownish to, as a result of inclusions of carbon of pyrolysis, blackish brown or, in the case of oxidation of the carbon caused by admission of air, also porcelain-like white.
The hollow beads are not siliceous, have good controllability of the chemical composition and the particle size and have a high heat resistance up to over 1200.degree. C.
Hollow beads of the invention can be produced by first drying, at from 20.degree. to 200.degree. C., reaction products of acid aluminium phosphates (and optionally boric acids) with amines, then comminuting them and finally subjecting them to a heat shock of from 200.degree. to 1200.degree. C. The reaction products of acid aluminium phosphates (and optionally boric acids) and mines can also be ones with amine salts.